Add migration that copies course_id -> course_key.

The migration copies content of the Answer.course_id column into the new
extended Answer.course_key column.

It skips Answer records that already have the course_key column set.

problem_builder/migrations/0004_copy_course_ids.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+def copy_course_id_to_course_key(apps, schema_editor):
+    """
+    Iterates over all Answer records for which course_key is not set
+    and copies the value of the course_id column into course_key.
+    """
+    Answer = apps.get_model('problem_builder', 'Answer')
+    for answer in Answer.objects.filter(course_key__isnull=True).iterator():
+        answer.course_key = answer.course_id
+        answer.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('problem_builder', '0003_auto_20161124_0755'),
+    ]
+
+    operations = [
+        migrations.RunPython(code=copy_course_id_to_course_key, reverse_code=migrations.RunPython.noop),
+    ]